What kind of keyboard are you using? If it is less than $20, buy a new one. It isn't worth cleaning out under the keys. If you are strapped for cash, canned air and a vacuum would be good. You could even dunk it in a tub of water and let it dry for a day or two.

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Canned air is air that is compressed into a can. It just shoots air at a moderate velocity to get the dust and dirt off. A vacuum cleaner should be fine, they make smaller ones for keyboards, but bigger ones should work the same. I'm a proponent to just drop it in water, make sure to remove the batteries and let it dry out for a day or two. I've done this when keyboards were actually expensive.

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Well, I'm very, very cheap and frugal, and I'll stretch out a penny every way I can. If that means saving $5 to buy a new keyboard, I will. I usually slam my keyboard on the desk and lots of stuff comes flying out.

I remember some water I spilled seeped under the keyboard, and I didn't notice any of it until three days later and my right arrow key stopped working.

The cleaner costs more than some old, ratty keyboards. It might be pretty nice to use on expensive wireless keyboards, though. But, if you have an old keyboard like me, you might as well chunk it and get a new wireless keyboard.

I always found it easy (but it depends on how many screws your keyboard has, mine has only 4) to just pop off the faceplate, take off the keys (but only if you know where they go when your done!) and then clean it. It takes 15-25 minutes and is pretty clean. I use a ratty old Compaq non-wireless keyboard on my main computer that I mostly keep for sentimental reasons. I only do this because I've never found any vacuums to work (not even the ones specifically designed to do so).
@Aron:
What about rust? Isn't that a factor?

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All I know is that dust is one of electronic's worst enemies. Why an avid computer user would not have "Canned Air" or other means of keeping their PC clean is beyond me.

You can pick up Canned or Compressed air" at Wal-Mart or any other Retail store in the Computer Dept..(it ain't rocket science)
I personally use a 25 dollar shop-vac that has the vacuum option as well as a blower option for blowing that build up of dust out of my computer case and off my fans etc.

I have an air compressor (a real one) that I use for blowing stuff out. We also have a shop vac (VERY USEFUL when dusting cases out so the dust gets sucked into the vacuum). The support stuff is important.

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Said By Aron Schatz

I have an air compressor (a real one) that I use for blowing stuff out. We also have a shop vac (VERY USEFUL when dusting cases out so the dust gets sucked into the vacuum). The support stuff is important.

Be careful with the Air Compressor. Depending on the pressure, you can actually blow a diode or other electronic component clean off your MOBO or Video card.
You can also blow the fans so hard that they will spin at an astonomical RPM which can damage the motor.
I don't personally recommend an actual Air Compressor unless you are experienced and somewhat careful.